Handle defered handling of exceptions (where the exception is thrown before the

Handle defered handling of exceptions (where the exception is thrown before the
handler is registered). Also, don't throw the exception on calls to then(), 
to avoid enforcing an ordering to the then() and handleException() calls.

BUG=1115
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=3477

[dcarlson, 2012-01-10 16:56:56]

Proposed change to the behavior of exception handling on futures.

[Siggi Cherem (dart-lang), 2012-01-10 20:39:25]

+mattsh

I think the change to 'handleException' makes sense: we want to preemptively
handle exceptions if we add the handler before calling [then]. However, the
change to [then] is more controversial (I added more comments on the bug:
http://code.google.com/p/dart/issues/detail?id=1115
)

http://codereview.chromium.org/9155017/diff/1/corelib/src/implementation/futu...
File corelib/src/implementation/future_implementation.dart (left):

http://codereview.chromium.org/9155017/diff/1/corelib/src/implementation/futu...
corelib/src/implementation/future_implementation.dart:88: } else if
(!_exceptionHandled) {
I believe we don't want to do this part of the change.

Basically we are worried about users of [Future] that don't register any
exception handlers at first (it wasn't initially part of their design), but
later end up inadvertently ignoring exceptions. 

It's unfortunate that this requires an ordering (add exception handlers before
calling 'then'), but I still believe exposing exceptions out-weights this.

http://codereview.chromium.org/9155017/diff/1/corelib/src/implementation/futu...
File corelib/src/implementation/future_implementation.dart (right):

http://codereview.chromium.org/9155017/diff/1/corelib/src/implementation/futu...
corelib/src/implementation/future_implementation.dart:92: if (null !==
_exception) {
(nit): keeping the style consistent with the rest of the dart code, swap the 2
expressions (_expression !== null)

[dcarlson, 2012-01-20 15:52:13]

This CL is reduced to focus on the agreed changes.

[Siggi Cherem (dart-lang), 2012-01-21 01:50:38]

lgtm

http://codereview.chromium.org/9155017/diff/6001/corelib/src/implementation/f...
File corelib/src/implementation/future_implementation.dart (right):

http://codereview.chromium.org/9155017/diff/6001/corelib/src/implementation/f...
corelib/src/implementation/future_implementation.dart:94: if (null !==
_exception) {
(swap): _expression !== null